Abstract The objective of the proposed research is to undertake a detailed analysis of an underinvestigated class of proteins: the phosphatidylinositol (PtdIns/ phosphatidylcholine (PtdCho) transfer proteins (PITPs). To this end, we will primarily employ the prototypical member of the Sec14-superfamily and its five yeast paralogs as experimental models. Our data indicate that the yeast PITP (Sec14) is an essential factor that operates at the interface of phospholipid metabolism and Golgi/endosomal membrane trafficking functions. The proposed studies will test specific hypotheses that relate to: (i) how the Sec14-like ?nanoreactor? proteins bind and exchange their lipid ligands, (ii) the mechanisms by which non-canonical Sec14-like proteins regulate specific steps of lipid metabolism in yeast, and (iii) how the oxysterol binding protein (OSBP)- related Kes1 works against Sec14-dependent PtdIns-4-phosphate signaling in regulating Golgi/endosomal membrane trafficking and how this lipid binding protein antagonism is played out in control of cell cycle progression through the G1 phase of the cell cycle. These studies will clarify key unanswered questions regarding the mechanism of function of the Sec14 itself, the mechanisms by which Sec14-like proteins couple lipid metabolism to PtdIns kinase signaling, and more global ramifications of PITP functional interactions with the oxysterol binding protein family members (ORPs). The available evidence suggests that PITPs and ORPs play central, and previously unrecognized, roles in lipid-mediated signal transduction processes that interface with such diverse cellular processes as membrane trafficking, basic lipid metabolism and cell cycle. A growing number of inherited neurodegenerative diseases, and diseases of proliferative disorders (e.g. cancer), are attributed to insufficiencies in PITPs and other Sec14-like proteins. Thus, the proposed studies will provide new and fundamental information that bears directly on molecular mechanisms by which PITPs, and Kes1-like OSBPs, regulate and organize signal transduction in eukaryotes and protect mammals from diseases of deranged cell proliferation and neurodegeneration.